Clotting of fresh whole blood is stimulated by adding calcium and tissue factor. Subsamples of the clotting blood are assayed for thrombin activity. The measured parameters are the maximal concentration of thrombin activity, the time to peak activity, and the integral of thrombin activity x time. We have tested the effect of hematocrit, white cell count, and platelet count on these parameters and have found that all of the cell types influence thrombin generation. We have also found that the concentration of each coagulation factor and natural inhibitor also affects thrombin generation in the assay system. [unreadable] [unreadable] The test has now been applied in a group of patients with advanced malignancies who undergo major surgery. Fourteen patients and 14 age- and sex-matched controls have been studied. There were obvious changes in thrombin generation between the pre-operative and post-operative days. Accrual for this project was halted prematurely because the surgeons now performing the operations prefer to give heparin prophylaxis pre-operatively. This timing confounds our measurements. Nevertheless, a large battery of tests has been completed to search for an explanation for the changes we observed between pre- and post-operative thrombin generation.